Preheat oven to 400°F with a middle rack. In a small bowl, mash 3 bananas with a fork, and then add the brown sugar, vanilla extract, and ground cinnamon. The fourth banana will be used later.
In a large mixing bowl, cream the butter and sugar until light and fluffy. Add one egg at a time, and continue to cream. Into the wet ingredient, sift in the flour, baking soda, and salt. Mix, but don't over mix, until all is combined.
Add the greek yogurt (use either plain or vanilla), and mix. Add the mashed banana mixture and walnuts (I like toasting the walnuts in the oven. It can also be eliminated for those with nut allergies).
In a 9x5 loaf pan, line it with parchment paper for easy removal. Alternatively, you can grease the loaf pan with butter. Pour mixture into the loaf pan.
Think of this bread loaf as a layered one. For the second layer, cut the last banana in half, length-wise. Place each piece on top of the loaf mixture. This layer adds additional creaminess. OPTION: You can bake it like this, OR add another layer of goodness!
For the final layer, in a small mixing bowl to prep for an amazing crumble topping, mix with your hand or a fork the flour, cinnamon, butter, and brown sugar. Sprinkle generously on top of the loaf.
Bake the loaf in the middle rack for 30-40 minutes. Insert a toothpick in the middle, and if it comes out clean, then it's done. Let it sit in the pan for 10 minutes, and then lift the wax paper, and place it on a cooling rack. OPTION: This freezes well for when company comes over: wrap with wax paper, and then insert in a ziplock bag!